The maroon-like mutants in Drosophila in conjuncton with mutations at several other separate loci are being studied as a system in order to question the regulation of gene expression in a higher eukaryotic form. This system includes maroon-like, a possible cofactor-producing gene, low xanthine dehydrogenase, probably a second cofactor-producing or regulator gene, and three structural genes whose expression is affected by mutations at the maroon-like and low xanthine dehydrogenase loci. Current work in progress seeks to isolate and characterize the product of the maroon-like locus as a first step in understanding the nature of its involvement with at least two other apparently unrelated genes.